lithographic apparatus, a projection system and a device manufacturing method

ABSTRACT

A lithographic apparatus is disclosed that includes a projection system configured to project a patterned radiation beam onto a target portion of a substrate, a vacuum chamber through which the patterned beam of radiation is projected during use, and a purge system configured to provide a purge gas flow in the chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application61/004,772, which was filed on 30 Nov. 2007, and which is incorporatedherein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus, a projectionsystem and a device manufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In a known lithography system, the projection system comprises a vacuumchamber wherein the patterned beam of radiation is projected. As such,the projected beam traverses at least a region of the vacuum chamber. Inthe vacuum chamber, contamination with particles, such as carbonhydroxyl particles originating from substrate structures, might damagean optical element, such as a mirror. Further, the contaminationparticles might influence the optical transmittance of the projectedbeam. Especially, an extreme ultraviolet (EUV) lithography system mightsuffer from such gas contamination.

In U.S. patent application publication no. U.S. Pat. No. 6,714,279, avacuum chamber is provided with an inert gas supply. By supplying aninert gas in the vacuum chamber contamination of optical components maybe suppressed.

SUMMARY

It is desirable, for example, to realize a lithographic apparatuswherein the occurrence of contamination particles in the vacuum chamberis further suppressed.

According to an aspect of the invention, there is provided alithographic apparatus comprising an illumination system configured tocondition a radiation beam, a support constructed to support apatterning device, the patterning device being capable of imparting theradiation beam with a pattern in its cross-section to form a patternedradiation beam, a substrate table constructed to hold a substrate, aprojection system configured to project the patterned radiation beamonto a target portion of the substrate, a vacuum chamber through whichthe patterned beam of radiation is projected during use, and a purgesystem configured to provide a purge gas flow in the chamber.

According to an aspect of the invention, there is provided a projectionsystem configured to project a patterned radiation beam onto a targetportion of a substrate in a lithographic apparatus, wherein theprojection system comprises a vacuum chamber wherein the patterned beamof radiation is projected during use and wherein the projection systemfurther comprises a purge system for providing a purge gas flow in thechamber.

According to an aspect of the invention, there is further provided adevice manufacturing method comprising projecting a patterned beam ofradiation onto a substrate, wherein a purge gas flow is applied in avacuum chamber through which the patterned beam of radiation isprojected.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a schematic view of a lithographic apparatus;

FIG. 3 depicts a schematic view of a lithographic apparatus according toan embodiment of the invention;

FIG. 4 depicts a chart of drag and gravity forces exerted on a particle;and

FIG. 5 depicts a schematic view of a contamination particle travelingalong a surface.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g.

UV radiation or visible light radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more patterning device supportstructures). In such “multiple stage” machines the additional tablesand/or support structures may be used in parallel, or preparatory stepsmay be carried out on one or more tables and/or support structures whileone or more other tables and/or support structures are being used forexposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery systemcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator and a condenser. The illuminator may be used to condition theradiation beam, to have a desired uniformity and intensity distributionin its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF2 (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor IF1 canbe used to accurately position the patterning device MA with respect tothe path of the radiation beam B, e.g. after mechanical retrieval from amask library, or during a scan. In general, movement of the supportstructure MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the support structure MT maybe connected to a short-stroke actuator only, or may be fixed.Patterning device MA and substrate W may be aligned using patterningdevice alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice MA, the patterning device alignment marks may be located betweenthe dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 depicts schematically a lithographic apparatus 1. Thelithographic apparatus comprises an illumination system 2, e.g.comprising an EUV source, that is configured to condition a radiationbeam. Further, the apparatus 1 comprises a support (not shown forclarity) configured to hold a patterning device 3, the patterning devicecapable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam. The apparatus 1 alsocomprises a substrate table 4 configured to hold a substrate. Thesubstrate can be moved by a substrate handler 5 so as to move thesubstrate for further processing along a substrate exchange path 6. Thelithographic apparatus 1 further comprises a projection system 7configured to project the patterned beam of radiation and a vacuumchamber 8 wherein the patterned beam of radiation is projected. Duringoperation of the lithographic apparatus 1, the projected patterned beam9 traverses a region of the vacuum chamber 8 before reaching thesubstrate that is held by the substrate table 4. The apparatus 1 alsocomprises a pump 10 configured to receive contamination particles thatare present in the vacuum chamber 8. In FIG. 1, an example contaminationparticle 11 is schematically shown. The contamination particle 11 movesin a quasi arbitrary path 12 in the vacuum chamber 8 before beingreceived in the pump 10.

FIG. 3 shows a schematic view of a lithographic apparatus 1 according toan embodiment of the invention. Similar to the apparatus 1 depicted inFIG. 2, the lithographic apparatus 1 according to an embodiment of theinvention comprises an illumination system 2, a support configured tohold a patterning device 3, a substrate table 4, a projection system 7and a vacuum chamber 8. Further, the projection system 7 of theapparatus 1 shown in FIG. 2 is provided with a purge system configuredto provide a purge gas flow in the vacuum chamber 8. By applying a purgegas flow in the vacuum chamber 8 wherein the patterned beam of radiationis projected, the number of contamination particles in the region wherethe projected beam 9 traverses the chamber 8 may be reduced, thushelping to improve the optical performance of the projection system 7.As the number of contamination particles reduces, the opticaltransmission may improve. Further, the life time of optical componentsmay increase.

In the embodiment shown in FIG. 3, the purge system comprises a purgeinlet 13 configured to supply a purge gas flow into the vacuum chamber8. The purge gas substantially follows a certain flow path 15,collecting contamination particles along the flow path 15. Further, thepurge system comprises two pumps 16, 17 arranged at side walls of thevacuum chamber 8 on opposite sides with respect to the section beingtraversed by the projected beam 9. The location of the purge inlet 13and the pumps 16, 17 is designed such that the purge gas substantiallyfollows the purge gas flow paths 15, 18. As an example, the gas flowpath 15 traverses the section of the vacuum chamber 8 that is traversedby the projected beam 9. A part of such a gas flow path 15 extends fromthe region traversed by the projected beam 9 towards the outside of thebeam of radiation so that contamination particles are removed from theregion, thus improving the optical performance of the apparatus 1.Another gas flow path 18 is situated along but outside the regiontraversed by the projected beam 9 and directed away from the projectedbeam 9, to drain contamination particles away from the projected beam 9.In FIG. 3, an example contamination particle 11 is shown following apath 14 directed to the pump 17. In contrast to the situation as shownin FIG. 1, the contamination particle is collected by the pump 17without traversing the projected beam 9, so that an opticaldeterioration of the apparatus 1 is counteracted. Further, ascontamination particles can be removed before they can cross thesubstrate exchange path 6, it may be advantageously counteracted that acontamination particle is stuck to the substrate table 4.

One or more further purge gas flow paths may be provided to removecontamination particles 11 from the vacuum chamber 8. As an example, afurther purge gas flow path from a section in a region in the chamber 8being traversed by the projected beam 9, towards the outside of theprojected beam 9, or a further purge gas flow path along but outside theprojected beam 9 region and directed away from the projected beam may beprovided. In principle, it is also possible to design only one type ofpurge gas flow path, e.g. a purge gas flow path extending substantiallytransversely with respect to the projected beam 9. Similarly, one ormore purge inlets and/or one or more pumps can be arranged forsubstantially determining a desired purge gas flow path.

According to an aspect of the invention, one or more purge inlets arearranged near a region that is critical for contamination particles, sothat contamination particles are more easily moved away, thuscounteracting the presence of so-called dead zones, i.e. regions wherecontamination particles can stay for a relatively long time periodbefore being removed.

According to a further aspect of the invention, one or more pumps arelocated desirably near a contamination particle source. As aconsequence, contamination particles can be removed from the vacuumchamber 8 relatively quickly before they can traverse the projected beam9.

According to an aspect of the invention, a hydrogen purge gas flow maybe applied, desirably in a region of the chamber that is traversed bythe patterned beam of radiation. Advantageously, hydrogen gas has arelatively good transmission for EUV radiation, so that the projectedbeam may be minimally disturbed by the hydrogen gas. Further, a halogenpurge gas flow may be applied in a region of the chamber outside thepatterned beam of radiation, to improve the pumping efficiency of thepumps. According to a further aspect of the invention, the purge gasflow may be provided along a surface of an optical device, to reduce thechance that contamination particles reach such an optical devicesurface.

A calculation of drag forces and gravity forces, respectively, exertedon contamination particles is described hereafter. A drag force exertedon a general mass particle in gas flow, and a gravity force exerted onthat mass particle, are given by respectively:

$\begin{matrix}{F_{d} = {{\left( \frac{8 + \pi}{3} \right) \cdot \sqrt{\frac{2{\pi \cdot M}}{R_{0} \cdot T}}}{r^{2} \cdot P \cdot u}}} & (1) \\{F_{g} = {{g \cdot \rho \cdot \frac{4}{3}}{\pi \cdot r^{3}}}} & (2)\end{matrix}$

wherein F_(d) represents a drag force exerted in the particle [N], Mdenotes a molar mass of the gas [kg/mol], R₀ denotes a gas constant[J/mol·K], T denotes the temperature [K], r denotes the radius of theparticle [m], P denotes a pressure [Pa] and u denotes a relativevelocity of the particle in the vacuum chamber [m/s]. Further, F_(g)represents a gravity force exerted on the particle [N] and p denotes thedensity [kg/m³] of the particle.

As an example, a steel particle having a radius of 100 nm in a hydrogenflow of 10 m/s at 10 Pa and 295K appear to have a drag force muchstronger than the gravity force. As a computational example, the dragforce and the gravity force according to equations (1) and (2) are:

$F_{d} = {{\left( \frac{8 + \pi}{3} \right)\sqrt{\frac{2{\pi \cdot 0.002}}{8.31 \cdot 295}}{\left( {50 \cdot 10^{- 9}} \right)^{2} \cdot 10 \cdot 10}} = {{2 \cdot 10^{- 15}}N}}$$F_{g} = {{{9.81 \cdot 7800 \cdot \frac{4}{3}}{\pi \cdot \left( {50 \cdot 10^{- 9}} \right)^{3}}} = {{2 \cdot 10^{- 17}}N}}$

FIG. 4 depicts a chart of drag and gravity forces 26 exerted on aparticle as a function of the particle diameter 25 of that particle. Inparticular, FIG. 4 shows a drag force F_(d) 19, 20, 21 assuming arelative velocity of the particle of 10 m/s, 1 m/s and 0.1 m/s,respectively. Similarly, FIG. 4 shows a gravity force F_(g) 22, 23, 24assuming a particle of steel, glass and nylon material, respectively.From the numerical results it can be deduced that the drag force exertedon relatively small particles is much greater than a gravity forceexerted thereon. As a result, such relatively small particles aregenerally “airborne” and will easily be moved by a gas flow. On theother hand, relatively heavy and large particles are subjected to agravity force that is greater than the drag force in a slow gas flow, sothat such particles are less sensitive to the applied purge gas flow. Ina practical situation, a gas flow speed can be chosen such thatcontamination particles that are present in the vacuum chamber can beremoved by the purge gas flow.

Contamination particles that are moved by the purge gas flow can diffusein a direction transverse with respect to the gas flow direction. Inorder to counteract a contamination particle from leaving a purge gasflow path, a time for flowing the gas along a critical surface should besmaller than a diffusion time for traversing the flow path. FIG. 5depicts a schematic view of a contamination particle 32 travelling in apurge gas flow path 31 along a surface 30. The particle 32 can follow adiffusion path 33 that is at least partly transverse with respect to adirection ΔY along which the surface 30 extends. A diffusion path lengthof the contamination particle can be calculated using the followingformulae:

$\begin{matrix}{\overset{\_}{\Delta \; X} = \sqrt{\frac{4 \cdot D \cdot t}{\pi}}} & (3) \\{D = \frac{{Cc} \cdot k \cdot T}{3 \cdot \pi \cdot \eta \cdot {Dp}}} & (4) \\{{Cc} = {1 + {\frac{\lambda}{Dp} \cdot \left( {2.514 + {0.8 \cdot {\exp \left( {- \frac{0.55 \cdot {Dp}}{\lambda}} \right)}}} \right)}}} & (5) \\{\lambda = \frac{k \cdot T}{\pi \cdot \sqrt{2} \cdot \sigma^{2} \cdot p}} & (6)\end{matrix}$

wherein ΔX represents a diffusion path length [m] within a time t [s], Ddenotes a diffusion velocity [m/s], Cc denotes a Cunningham slip factor,k denotes the Boltzman constant [J/K], T denotes the temperature [K], θdenotes the viscosity [Pa·s], Dp denotes the particle diameter [m], λdenotes the mean free path [m], σ denotes the gas molecule cross sectionand p denotes the pressure [Pa].

As an example, a contamination particle having a particle diameter of 60nm in a gas flow of 10 m/s along a surface of 20 cm length in adirection ΔY transverse with respect to a diffusion length ΔX gives anaverage residence time of about 0.02 s. Within the time period of 0.02s, the average diffusion length at 10 Pa, 295K hydrogen (σ=0.29 nm) isabout 1 mm. In a practical example, therefore, by providing a purge gasflow having a velocity>10 m/s having a laminar behavior, meaning thatthe Reynolds number is smaller than approximately 2300, which conditionis met in the transition area between molecular flow and viscous flowsince the velocity is limited by the speed of sound and the flow islaminar, contamination particles in a purge gas flow path that is offsetat least 1 cm from a critical surface, can in principle not reach thecritical surface by diffusion. By applying an offset that is at leasttwo times the average diffusion length, it can be counteracted thatcontamination particles reach a critical surface by diffusion. Further,in order to obtain a viscous flow, the Knudson parameter Kn is smallerthan 1, desirably much smaller than 1. It is noted that both the averagediffusion length and the free path length are dependent on the pressure.As an example, the average diffusion length is 1 mm and 3 mm at 10 Paand 1 Pa, respectively. Similarly, the free path length is 1 mm and 10mm at 10 Pa and 1 Pa, respectively, meaning that the purge flow can notbe regarded as a viscous flow at 1 Pa.

According to a further aspect of the invention, the temperature of thepurge gas flow is relatively cold. In an embodiment, the temperature ofthe purge gas flow is colder than the temperature of a surface of anoptical device along which the purge gas flow is applied. As a result ofa thermophoretic effect, contamination particles may be more easily keptaway from the optical device surface.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: an illumination systemconfigured to condition a radiation beam; a support constructed tosupport a patterning device, the patterning device being capable ofimparting the radiation beam with a pattern in its cross-section to forma patterned radiation beam; a substrate table constructed to hold asubstrate; a projection system configured to project the patternedradiation beam onto a target portion of the substrate; a vacuum chamberthrough which the patterned beam of radiation is projected during use;and a purge system configured to provide a purge gas flow in thechamber.
 2. A projection system configured to project a patternedradiation beam onto a target portion of a substrate in a lithographicapparatus, wherein the projection system comprises a vacuum chamberwherein the patterned beam of radiation is projected during use andwherein the projection system further comprises a purge systemconfigured to provide a purge gas flow in the chamber.
 3. The projectionsystem of claim 2, wherein the purge system comprises a purge inletconfigured to supply a purge gas flow into the vacuum chamber.
 4. Theprojection system of claim 3, wherein the purge inlet is arranged near aregion that is critical for contamination particles.
 5. The projectionsystem of claim 2 wherein the purge system further comprises a pumpconfigured to receive the purge gas flow.
 6. The projection system ofclaim 5, wherein the pump is near a contamination particle source.
 7. Adevice manufacturing method comprising projecting a patterned beam ofradiation onto a substrate, wherein a purge gas flow is applied in avacuum chamber through which the patterned beam of radiation isprojected.
 8. The method of claim 7, comprising further providing apurge gas flow path along but outside the patterned beam of radiationand directed away from the patterned beam of radiation.
 9. The method ofclaim 7, comprising further providing a purge gas flow path from asection in a region of the chamber that is traversed by the patternedbeam of radiation, towards the outside of the patterned beam ofradiation.
 10. The method of claim 7, comprising applying a hydrogenpurge gas flow.
 11. The method of claim 10, comprising applying thehydrogen purge gas flow in a region of the chamber that is traversed bythe patterned beam of radiation.
 12. The method of claim 7, comprisingapplying a halogen purge gas flow in a region of the chamber outside thepatterned beam of radiation.
 13. The method of claim 7, furthercomprising providing a purge gas flow along a surface of an opticaldevice.
 14. The method of claim 13, wherein the temperature of the purgegas flow provided along the surface of the optical device is colder thanthe temperature of the surface of the optical device.